1. Field of the Invention
This invention generally relates to coatings for implantable electrodes such as pacing electrodes, neurostimulator electrodes, and electroporating electrodes and sensing electrodes. The three overriding requirements for these types of electrodes are biocompatibility, biostability, and low energy loss. Broadly, the biocompatibility requirement is met if contact of the electrode with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting) and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that all physical, electrical, and chemical properties of the electrode/coating system remain constant and unchanged over the life of the patient. The low energy loss requirement is met if electrode polarization is a minimum.
2. Prior Art
U.S. Pat. No. 4,602,637 to Elmqvist describes a commonly used pacing electrode having sputtered columnar titanium nitride as a coating material. This form of titanium nitride has good conductivity combined with a high specific surface area, resulting in favorable polarization and sensing properties. The disadvantage of titanium nitride, however, is that it degrades the electrical properties of surrounding tissue after implantation. This occurs as the body tissue encapsulates the columnar titanium nitride in fibrotic tissue, which has a lower conductivity than normal tissue.
In the case of a pacing electrode, fibrotic tissue raises the stimulation threshold. The stimulation threshold is the minimum energy required to produce a cardiac contraction. This, in turn, impacts the battery life of the system so that the medical device must be explanted sooner than desired. The encapsulation process also interferes with sensing of intrinsic milivolt signals required by pacemakers. In prior electrode designs, the fibrotic encapsulation problem has been addressed by incorporating a means of metering or eluting steroidal medication to the site of tissue contact over time. However, eluting a steroidal medication to the implant site is not completely effective in eliminating the stimulation threshold rise due to encapsulation. Steroidal medication eluting arrangements have a short duration of effectiveness, and also add cost and complexity to the system, add the risk of infection, and, in many cases, a portion of the electrode working surface must be dedicated to the medication administering function.
Other efforts to overcome the problem of fibrotic encapsulation are described in U.S. Pat. No. 4,495,039 to Cerise et al. and U.S. Pat. No. 4,612,100 to Edeling et al. The former patent relates to electrodes made of pyrolytic carbon while the latter is directed to electrodes coated with amorphous sputtered carbon. These designs meet the requirement of improved biocompatibility, but they do not have the high specific surface characteristics of columnar titanium nitride, and so fall short in polarization and sensing properties.